Author Topic: A Fuzzy logic DC-Link Voltage Controller for Three-Phase DSTATCOM to Compensate  (Read 2731 times)

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Author : Kranthi Kumar Kora
International Journal of Scientific & Engineering Research Volume 2, Issue 10, October-2011
ISSN 2229-5518
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Abstract- A fuzzy Logic controls the transient response of the distribution static compensator (DSTATCOM) is very important while compensating rapidly varying unbalanced and nonlinear loads. Any change in the load affects the dc-link voltage directly. The sudden removal of load would result in an increase in the dc-link voltage above the reference value, whereas a sudden increase in load would reduce the dc-link voltage below its reference value. The proper operation of DSTATCOM requires variation of the dc-link voltage within the prescribed limits. Conventionally, a proportional-integral (PI) controller and Fast-acting controller is used to maintain the dc-link voltage to the reference value. It uses deviation of the capacitor voltage from its reference value as its input. However, the transient response of the conventional PI dc-link voltage controller is slow,the transient response of the Fast-Acting DC link voltage is moderate. In this paper, a fuzzy logic dc-link voltage controller based on the energy of a dc-link capacitor is proposed. Mathematical equations are given to compute the gains of the conventional controller based on fast-acting dc-link voltage and fuzzy logic dc-link voltage  controllers to achieve similar fast transient response. The detailed simulation and experimental studies are carried out to validate the proposed controller.

Index Terms- DC-link voltage controller, distribution static compensator (DSTATCOM), fast transient response, harmonics, load compensation, power factor, power quality (PQ), unbalance, voltage-source inverter (VSI),fuzzy logic(FL).

The proliferation of power-electronics-       based equipment, nonlinear and unbalanced loads, has aggravated the power-quality (PQ) problems in the power distribution network. They cause excessive neutral currents, overheating of electrical apparatus, poor power factor, voltage distortion, high levels of neutral-to-ground voltage, and interference with communication systems [1], [2]. The literature records the evolution of different custom power devices to mitigate the above power-quality problems by injecting voltages/currents or both into the system [3]–[6].
      The shunt-connected custom power device, called the distribution static compensator (DSTATCOM), injects current at the point of common coupling (PCC) so that harmonic filtering, power factor correction, and load balancing can be achieved. The DSTATCOM consists of a current-controlled voltage-source inverter (VSI) which injects current at the PCC through the interface inductor. The operation of VSI is supported by a dc storage capacitor with proper dc voltage across it.
   One important aspect of the compensation is the extraction of reference currents. Various control algorithms are available in literature [7]–[11] to compute the reference compensator currents. However, due to the simplicity in formulation and no confusion regarding the definition of powers, the control algorithm based on instantaneous symmetrical component theory [11] is preferred.

Based on this algorithm, the compensator reference currents 〖(i〗_fa^*,i_(fb,)^* i_fc^*) are given as fallows.
i_fa^*= i_la-(v_sa+ γ(v_sb-v_sc ))/(∑_(i=a,b,c)▒v_si^2 )(p_1avg+p_dc)
i_fb^*= i_lb-(v_sb+ γ(v_sc-v_sa ))/(∑_(i=a,b,c)▒v_si^2 )(p_1avg+p_dc)
i_fc^*= i_lc-(v_sc+ γ(v_sa-v_sb ))/(∑_(i=a,b,c)▒v_si^2 )(p_1avg+p_dc)
 Where γ=tan∅/√3,∅ is the desired phase angle between the supply voltages and compensated source currents in the respective phases. For unity power factor operation, ∅=0, thus γ=0. The term p_1avgis the dc or average value of the load power. The term p_dcin (1) accounts for the losses in the VSI without any dc loads in its dc link. To generate p_dc , a suitable closed-loop dc-link voltage controller should be used, which will regulate the dc voltage to the reference value.
   For the DSTATCOM compensating unbalanced and nonlinear loads, the transient performance of the compensator is decided by the computation time of average load power and losses in the compensator. In most DSTATCOM applications, losses in the VSI are a fraction of the average load power. Therefore, the transient performance of the compensator mostly depends on the computation of p_1avg. In this paper,p_1avg is computed by using a moving average filter (MAF) to ensure fast dynamic response. The settling time of the MAF is a half-cycle period in case of odd harmonics and one cycle period in case of even harmonics presence in voltages and currents. Although the computation of〖 p〗_dc is generally slow and updated once or twice in a cycle, being a small value compared top_1avg, it does not play a significant role in transient performance of the compensator.
   In some of the electric power consumers, such as the telecommunications industry, power-electronics drive applications, etc., there is a requirement for ac as well as dc loads [12]–[15]. The telecommunication industry uses several parallel-connected switch-mode rectifiers to support dc bus voltage. Such an arrangement draws nonlinear load currents from the utility. This causes poor power factor and, hence, more losses and less
efficiency. Clearly, there are PQ issues, such as unbalance, poor power factor, and harmonics produced by telecom equipment in power distribution networks. Therefore, the functionalities of the conventional DSTATCOM should be increased to mitigate the aforementioned PQ problems and to supply the dc loads from its dc link as well. The load sharing by the ac and dc bus depends upon the design and the rating of the VSI. This DSTATCOM differs from conventional one in the sense that its dc link not only supports instantaneous compensation but also supplies dc loads.
   However, when the dc link of the DSTATCOM supplies the dc load as well, the corresponding dc power is comparable to the average load power and, hence, plays a major role in the transient response of the compensator. Hence, there are two important issues. The first one is the regulation of the dc-link voltage within prescribed limits under transient load conditions. The second one is the settling time of the dc–link voltage controller. Conventionally, a PI controller and Fast-Acting controller is used to maintain the dc-link voltage. It uses the deviation of the capacitor voltage from its reference value as its input. However, the transient response of the conventional dc-link voltage controllers is slow,the transient response of fast-acting dc-link voltage controller is moderate, especially in applications where the load changes rapidly. Some work related to dc-link voltage controllers and their stability is reported in [16]–[20]. However, the work is limited to rectifier units where switching patterns are well defined and analysis can be easily carried out. In this paper, a fuzzy logic dc-link voltage controller based on the dc-link capacitor energy is proposed. The detailed modeling, simulation, and experimental verifications are given to prove the efficacy of this fuzzy logic dc-link voltage controller. There is no systematic procedure to design the gains of the conventional PI controller and fast-acting controller used to regulate the dc-link voltage of the DSTATCOM. Herewith, mathematical equations are given to design the gains of the conventional controller based on the fast-acting and fuzzy logic dc-link voltage controllers to achieve similar fast transient response.

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